The invention relates to an oil-cooled cylinder head for a four-stroke internal combustion engine in which combustion occurs with little heat loss. Combustion takes place in a hot core and this core is surrounded by an insulating layer of excess air. Accordingly, the cylinder wall requires no external cooling, at least not below the area of the piston which serves as the ring carrier.
The subject of the present application is thus an improvement for the cooling system of a four-stroke internal combustion engine, especially the cylinder head. Among other things, the improvement brings about savings in fuel by reducing cooling requirements, decreases frictional losses between the piston and the cylinder wall, and eliminates thermal stresses.
The subject of the present invention is also a conversion kit for modifying water-cooled or air-cooled internal combustion engines on the market so that the improved thermal characteristics of an oil-cooled engine can be imparted to the same.
Improvements in the thermal characteristics of an engine cannot be achieved by cooling the parts adjacent to the combustion chamber as intensively as possible. Nevertheless, this has been the conception for the past several decades. This is indicated by the use of water as the usual cooling medium in practice. Due to its high heat capacity, water constitutes a reliable agent for the optimal removal of heat from the engine. However, this reliability is associated with the drawback that a separate cooling circuit, including flow channels in the cylinder head and motor block, must be provided. Thus, although water is not to contact functional parts of the engine and is inherently foreign to engine operation, it has been employed because of its cooling efficiency. As a result, instead of designing the engine for optimum operation, the overall thermal characteristics of the engine were based on the properties of water as a cooling agent; the effect on the development of engine components was, for example, that strongly heat-conducting materials such as aluminum were used for pistons, cylinders and even cylinder heads so that the intense cooling action of the water could penetrate to the interior of the engine.
Thermal cracks in the material are only one phenomenon which demonstrate the contrast between engine operation with hot gases and abrupt cooling of the material to the temperature of the cooling water. A contributing factor here is that the possibility of rapid heat removal leads to combustion processes whose performance is based on heat removal through the walls of the combustion chamber, that is, all thermally uniform combustion processes in which the entire quantity of air is mixed with the fuel and the walls of the combustion chamber are subjected to a flame upon combustion. This includes chamber processes which are associated with significant heating of the combustion chamber walls.
To the present, the drawback of this cooling concept is that up to a third of the energy supplied is lost to the cooling water and, from there, must be transferred to the atmosphere via heat exchangers. It is no different in engines with external air cooling where numerous cooling ribs are provided in an attempt to make the engines as effective as a water-cooled engine with respect to heat removal. Experiments with oil cooling, which are carried out now and then, also function in accordance with this principle: it is attempted to compensate for the lower heat capacity of oil compared to water by increasing the flow quantity and the sizes of the oil cooling chambers with reference to water-cooled engines. From the point of view of rapid heat removal, oil cooling cannot compete with water. Since the oil pumping devices must be enlarged, the operating efficiency of the engine is reduced.
It would also be incorrect to incorporate ceramic parts in a combustion chamber as a result of a development which had heat removal as its Primary Seal. Although the intent is to provide heat insulation since, the walls of the combustion chamber are heated during thermally uniform combustion processes, these walls become the starting point for combustion so that the last natural insulating layer, namely, the boundary air layer between ceramic part and combustion chamber, is destroyed. When this last air insulator between hot gases and wall is eliminated, heat transfer at the material increases abruptly instead of decreasing as originally contemplated.
In all of these cooling concepts, the fundamental reason for the concern with the thermal characteristics of engines has been neglected. This is that the heat of combustion of the fuel is to be used for heating of the working air, so that the latter expands and moves the pistons, and not for heating of the engine, other components or water.
Only the periodic increases in energy costs and subsequent attention to inexpensive fossil fuels brought about a recollection of the actual purpose of heating the air in the cylinders. This led to the objective of foregoing the concept of external cooling, whether with water or ambient air, in favor of achieving higher efficiencies. When vegetable oils, which impose much stricter requirements on the thermal characteristics of engines, began to be used as fuels, it became even more necessary to accelerate such a development.
It was not sufficient, however, to merely modify the incorrect conception of intensive heat removal which, instead, had to be rethought entirely; the present text also contributes to this rethinking. Thus, if heat removal from inside to outside is to be prevented, a whole series of measures must be considered for the region extending from the center of heating, namely, the combustion zone, through the pistons and to the cylinders.
The first step was the creation of a duothermal combustion process in contrast to the thermally uniform process which is still customarily used today in the reciprocating internal combustion engine, chamber engine and D.I. diesel engine. Thus, a layer of air is maintained between the central combustion zone and the wall of the combustion chamber. This layer does not participate in the combustion and, due to its insulating properties, allows only small quantities of heat to pass to the walls of the combustion chamber (West German Offenlegungsschrift No. 22 41 355 and the corresponding U.S. Pat. No. 4,015,577). If only small quantities of heat are transmitted to the walls of the combustion chamber, especially the pistons, there is no longer any need to make these of aluminum. It is possible to once again make the pistons of an iron alloy. An iron alloy, in turn, exhibits lower conductivity for heat still arriving at the pistons. These actions are accompanied by a third which resides in that the heat transfer areas between piston and cylinder are minimized; the ring carrier is separated from the skirt of the piston and, in the remaining upper portion of the piston, the heat transfer areas between the ring carrier and the bottom of the piston are further reduced (e.g., West German Offenlegungsschrift No. 32 10 771 and the corresponding U.S. Pat. No. 4,593,660). By virtue of the heat insulating measures described, this state of the art resulted in a minimization of the overall cooling requirements for engines and cooling was carried out, without any problems related to the low heat capacity of oil, using oil as a cooling agent (West German Offenlegungsschrift No. 33 14 543 and the corresponding U.S. Pat. No. 4,895,111). Spray cooling of the cylinder walls and piston with oil from below plays an important role here.
One of the questions which arises with the given state of development of oil-cooled engines is whether it would not also be possible, either retroactively or by engine interchange, to convert current water-cooled or air-cooled four-stroke internal combustion machines available on the market to the new oil cooling. Since the new oil cooling offers advantages with respect to fuel consumption and the use of alternative fuels such as, for example, vegetable oils, such a conversion can be worthwhile.